A critical barrier to overcoming triazole resistance in Aspergillus fumigatus is the significant lack of understanding of its genetic and molecular basis. We have shown that the known mechanisms of resistance do not fully explain resistance observed among most clinical isolates. Our long-term goal is to improve antifungal therapy and ensure the sustained clinical utility of the triazole class for treatment of infections caused by Aspergillus species. Our central hypothesis is that non-cyp51A-mutation mediated mechanisms are essential to triazole resistance in clinical isolates of A. fumigatus and involve complex genetic changes altering 1) sterol biosynthesis and its transcriptional activation, 2) triazole transport and its transcriptional activation, and 3) as yet unknown mechanisms. Our current objective is to address critical knowledge gaps by identifying the genetic and molecular determinants of non-cyp51A-mutation mediated resistance. Our preliminary data suggest that while mutations in cyp51A among triazole resistant clinical isolates are common, their overall contribution to resistance is minimal. We have observed mutations, unique to resistant isolates in our collection, in genes encoding sterol sensing proteins, regulators of sterol biosynthesis, and sterol biosynthesis enzymes. We have also observed clinical isolates that overexpress not only cyp51A, but most genes of the ergosterol biosynthesis pathway, suggesting its constitutive activation. We have observed several potential transporters that are up- regulated among triazole resistant isolates in our collection, suggesting a role for triazole efflux and resistance by these transporters. We have also shown that clinical isolates of A. fumigatus take up triazole antifungals via facilitated diffusion and we believe that altered triazole import may represent an important mechanism of resistance. To accomplish our objective we will undertake experiments that will lead to an understanding of what genetic and molecular determinants influence triazole susceptibility through altered sterol biosynthesis or its transcriptional activation (Aim 1) and triazole transport and its regulation (Aim 2). In Aim 3, we will also utilize an unbiased whole genome comparisons, coupled with in vitro evolution experiments, to identify completely novel mechanisms of resistance in clinical isolates. Our approach is innovative as we will use the latest genetic and genomic techniques to study and discover novel non-cyp51A-mutation mediated mechanisms of triazole resistance that are operative in a U.S.-based collection of triazole resistant clinical isolates. The proposed research is significant as it represents a comprehensive analysis of the molecular and genetic basis of non- cyp51A-mutation mediated triazole resistance in A. fumigatus and will provide novel insights into ways in which triazole activity can be improved against this important human pathogen.